The Extent of Equilibration between Melt and Residuum during Regional Anatexis and its Implications for Differentiation of the Continental Crust: a Study of Partially Melted Metapelitic Enclaves

摘要

This study presents the first precise evaluation of the extent of chemical equilibrium between felsic melt and crystalline residuum during the crustal anatexis of metasediments. The high precision of these results stems from the fact that, in this case, the melts are represented by pristine glasses that occur in the matrices of partially melted metapelitic enclaves, and as abundant glassy melt inclusions trapped by different minerals in the same enclaves. In previous studies of anatexis, the composition of the melt has been approximated by that of leucosomes in migmatites, which do not necessarily represent the composition of the melt very well. The present study is based on laser ablation analyses of 35 trace elements in the glass and principal minerals of enclaves included within Neogene peraluminous dacites exposed at El Hoyazo, SE Spain. The enclaves, which contain the assemblage plagioclase + biotite + sillimanite + garnet + glass + ilmenite + graphite ± K-feldspar ± cordierite ± quartz, represent fragments of metapelitic continental crust that was partially melted at pressures of 5–7 kbar during a regional metamorphic event. Matrix melt and melt inclusions within the enclaves were quenched to glass upon eruption of the dacite. The glasses in the melt inclusions are interpreted as the remains of melt produced by an initial melting reaction involving a muscovite-rich assemblage at ≈700–750°C, whereas the matrix glasses are consistent with continued melting and/or a later melt dominated by the incipient melting of biotite at ≈800–850°C. The distribution of trace elements between residuum and melt during anatexis of the enclaves is different from that predicted by disequilibrium models that consider effective distribution coefficients close to unity. Instead, the bulk melt and the major minerals (plagioclase, biotite, alkali feldspar, cordierite) were close to equilibrium (except for the case of garnet) during the generation of the melt inclusions and matrix glasses. The quenched melts (melt inclusions and particularly the matrix glasses) were not in equilibrium with accessory zircon and monazite, and are depleted in zirconium and the light rare earth elements. Recrystallization of minerals and diffusion in the melt were important controls during the redistribution of trace elements between the solids and the bulk melt. Local disequilibrium processes also took place at mineral–melt interfaces during the crystallization of peritectic minerals and the recrystallization of residual phases, as shown by the depletion of compatible elements in the melt inclusions.